Electrically excited flowing gas laser and method of operation

ABSTRACT

A method of and apparatus for producing spatially uniform discharges including producing laser action in a flowing gas by electrical means using first means to create electrons and second means to maintain the optimum electron environment to produce lasing action. Apparatus for and a method of producing spatially uniform discharges substantially throughout a large volume of gaseous medium by generating in an enclosure a substantially uniform density of free electrons in the medium and controlling the electron temperature of the free electrons to increase their average energy without substantially increasing their density that at a predetermined level and uniformity of both the density and temperature of the medium, a stable and uniform discharge is produced in the medium suitable for the intended use of the medium such as the generation and amplification of light waves by means of devices including gaseous media in which stimulated emission of radiation is provided by electrical means to create free electrons and maintain the optimum electron environment to produce lasing action, there being a particular relationship between the density of free electrons, gas pressure and gas velocity.

[ 51March 20, 1973 [57] ABSTRACT A method of and apparatus for producingspatially uniform discharges including producing laser action in aflowing gas by electrical means using first means to create electronsand second means to maintain the optimum electron environment to producelasing action.

Apparatus for and a method of producing spatially uniform dischargessubstantially throughout a large volume of gaseous medium by generatingin an enclosure a substantially uniform density of free electrons in themedium and controlling the electron temperature of the free electrons toincrease their average energy without substantially increasing theirdensity that at a predetermined level and uniformity of both the densityand temperature of the medium, a stable and uniform discharge isproduced in the medium suitable for the intended use of the mediumsuch'as the generation andamplification of light waves by means ofdevices including gaseous media in which stimulated emission ofradiation is provided by electrical means to create free electrons andmaintain the optimum electron environment to produce lasing action,there being a particular relationship between the density of freeelectrons, gas pressure and gas velocity.

50 Claims, 8 Drawing Figures TO SUSTAlNER CIRCUIT 46 Mass.

Cincinnati, Ohio .331/94.5 PE .H0ls 3/00 .331/94.5

ELECTRICALLY EXCITED FLOWING GAS LASER AND METHOD OF OPERATION Inventor:James P. Reilly, Lexington Filed: June 39,1970

[21] Appl. No.: 50,933

Related US. Application Data [63] Continuation-impart of Ser. No.859,424, Sept. 19,

1969, abandoned.

References Cited UNITED STATES PATENTS 9/1968 11/1970 Wilson 11/1967Goldsmith et a1. 45 PowEa SUPPLY SUSTAINER CIRCUIT United States PatentReilly [73] Assignee: Avco Corporation,

[52] 11.8. 51 [58.] Field of 1 Primary Examiner-Ronald L. WibertAssistant ExaminerConrad Clark Attorney-Charles M. Hogan and Melvin E.Frederick GAS EXHAUST PULSER CIRCUIT HIGH VOLTAGE POWER SUPPLY TOSUSTAINER ELECTRODES 2| GAS SOURCE LASER OUTPUT PATENTEDmzmm SHEET 1 BF4 9 E3050 mmzzkwzm OP momDOm m w JAMES P REILLY INVENTOR.

BY W

ATTORNEYS PATENTEUNARZOIHTS SHEET 20F 4 GAS SOURCE a e e 0 0 Q 9 a o a o6 0 0 0 Q a m @IIIIY Q 0 Q a e a a a e /P b a 6 N e l a a Q )5 VSUSTAINER CIRCUIT POWER SUPPLY BY M GAS FLOW u l E R R S E M A J O A W UEY C G m MW C U R OS E VR 8 H L W P 8 3 ATTORNEYS PATENTEUHARZOIEJHSHEET 3 OF 4 2 N E R U P PURE CO E volts lcm /N per? /cm 5 N VIBRATIONALEXCITATION CO 00! STATE EXCITATION CO OIO STATE EXCITATION N2 IONIZATIONRATE 3x .2528 MEE JAMES P REILLY ELECTRON TEMP INVENTOR MAXWELL AVG'D (ZZ e v BY ATTORNEYS ELECTRICALLY EXCITED FLOWING GAS LASER AND METHOD OFOPERATION This application is a continuation-in-part of application Ser.No. 859,424, filed Sept. 19, 1969 now abandoned.

Apparatus for and the method of producing a discharge in accordance withthe invention is useful for, but not limited to, the production oflasing action, electrically conductive ionized gas for use inmagnetohydrodynamic (MI-ID) devices and the like, or to produce chemicalreactions.

The present invention in its broadest sense is directed to theproduction of and apparatus for providing spatially uniformuseful'discharges in a gas at pressure levels and volumes such thatelectron pair diffusion to confining walls is negligible, that is, thedischarge is not wall dominated.

In a preferred embodiment, the invention comprises a high-power flowinggas laser which is volumetric in character and that can be scaled in allthree characteristic dimensions as well as in pressure level. A spatially uniform discharge is created where electron-ion diffusion to thewalls is negligible.

While the preferred embodiment of the present invention will bedescribed in connection with a flowing electrically excited nitrogen (Ncarbon dioxide (CO and helium (I-Ie) laser, it may, as noted above byway of example, be applied to other systems where a conducting ionizedgas is required or useful and including, but not restricted to, gasconstituents other than N CO and Be as well as other lasing systems. Ina lasing environment, a discharge in accordance with the invention hasthe correct electron temperature for most efficient laser operation.Moreover, a laser in accordance with the invention is volumetric in thesense that the proper gas temperature and lower laser stateconcentrations are maintained by means other than diffusion through thegas to cooled side walls.

Light amplification by stimulated emission of radiation (laser) hasextended the range of controlled electi'omagnetic radiation to theinfrared and visible light spectrum. A laser produces a beam of coherentelectromagnetic radiation having a particular well-defined frequency inthat region of the spectrum broadly described as optical. This rangeincluded the near ultraviolet, the visible and the infrared. Thecoherence of the beam is particularly important because it is thatproperty which distinguishes laser radiation from ordinary opticalbeams. On account of its coherence, a laser beam has remarkableproperties which set it apart from ordinary light which is incoherent.While the maser (microwave amplification by stimulated emission ofradiation) and the laser are based on the same principles of statisticaland quantum mechanics, the problems and the physical embodiments forachieving laser action are completely different from those for masers.

Coherence, the essential property of lasers is of two kinds: spatial andtemporal. A wave is spatially coherent over a time interval if thereexists a surface over which the phase of the wave is the same (or iscorrelated) .at all points. A wave is time-coherent at an infinitesimalarea on a receiving surface if there exists a periodic relationshipbetween its amplitude at any one instant and its amplitude at laterinstants of time. Perfect time coherence is an ideal since it impliesperfect monochromaticity, something which is forbidden by theuncertainty principle.

Laser beams have a number of remarkable properties. Because of theirspatial coherence, they have an extremely small divergence and aretherefore highly directional. For example, a ruby laser beam one inch indiameter at the source will be about four feet across on a surface tenmiles away. The very best that could be accomplished over the samedistance with an incoherent source, such as an arc lamp at the focus ofa 6-foot parabolic mirror, would be a beam spread over an area more thanone-third of a mile across. Another important feature of lasers is theenormous power that can be generated in a very narrow wave length range.Under certain operating conditions, monochromatic bursts of millions ofwatts can be produced. To get comparable radiation intensity from ablack body, it would have to be raised to a temperature of hundreds ofmillions of degrees-a condition not practically achievable. A laserbeam, because it possesses space coherence, can be focused to form aspot whose diameter is of the order of one wave length of the laserlight itself. Enormous power densities are thus attainable. For example,the focused output of a 50-kilowatt infrared burst from a laser can havea radiant power density of the order of 10 watts/cm; this is aboutmillion times the power density at the surface of the sun.Extraordinarily high temperatures, orders of magnitude greater than thatat the sun, can be generated at the small area which absorbs thisconcentrated radiation. Furthermore, since the electric field strengthof an electromagnetic wave is proportional to the square root of itsintensity, the field at the focus of the laser beam can be millions ofvolts per centimeter. The most promising potential of lasers comes fromtime coherence. It is this property which permitted prior artexploitation of radio and microwaves for communications. However, laserfrequencies are millions of times higher than radio frequencies, andhence are capable of carrying up to millions of times more information.In fact, one single laser beam has in principle moreinformation-carrying capacity than all the combined radio and microwavefrequencies in use at the present time.

Accordingly, systems applications of lasers are useful for communicationin space, on earth, and under sea. Military surveillance and weaponssystems, mapping, medical, and computer technology may also includelasers.

Two conditions must be fullfilled in order to bring about laser action:(1) population inversion must be achieved and (2) an avalanche processof photon amplification must be established in a suitable cavity suchas, for example, an optical cavity. Population inversion,

can be accomplished if (1) the atomic system has at 7 least three levels(one ground and at least two excited levels) which can be involved inthe absorption and emission processes and (2) the spontaneous emissionlifetime of one of the excited states is much longer than that of theother or others.

When a system is in a condition where light (photon) amplification ispossible, laser action can be achieved by providing (1 means forstimulating photon emission from the long-lived state, and (2) means forcausing photon amplification to build up to extremely high values. Inthe usual embodiment, this is accomplished by fashioning the mediumcontaining the active atoms into a cylinder with perfectly (as far aspossible) parallel ends polished so highly that the surface roughness ismeasured in terms of small fractions of a wave length of light. The endsare silvered or dielectric coated to behave as mirrors which reflectphotons coming toward them from the interior of the cylinder. Such astructure, whether the mirrors are within or outside the container, iscalled an optical cavity. If now pumping means, such for example anintense source, acts on the medium and brings about population inversion.of the long-lived state with respect to another lower energy excitedstate even though the long-lived state is only relatively longlived, ina small fraction of a second there will be spontaneous emission ofphotons. Most of these photons will be lost to the medium but some ofthem will travel perpendicular to the ends and be reflected back andforth many times by the mirrors. As' these photons traverse the activemedium, they stimulate emission of photons from all atoms in thelong-lived state which they encounter. In this way the degree of lightamplification in the medium increases extraordinarily and because thephotons produced by stimulated emission have the same direction andphase as those which stimulate them, the electromagnetic radiation fieldinside the cylinder or cavity is coherent. in order to extract a usefulbeam of this coherent light from the cavity, one (or both) of themirrors is made slightly transmissive. A portion of the highly intensebeam leaks through the mirror, and emerges with plane-parallel,regularly spaced wavefronts. This is the laser beam.

Parallelism of the mirrors is a rigorous geometrical requirement. If themirrors are not precisely parallel, the light rays that build up in thecavity will tend to digress further and further toward the edges of themirrors as they are reflected back and forth between the mirrors, andfinally they will be directed out of the cavity altogether. It isessential that any deviation from parallelism be so small that thecoherent photon streams will reflect back and forth a very large numberof times to build up the required intensity for laser action. In aconventional well-made laser cavity the angle that one mirror makes withthe other (called wedge angle) is about 2 seconds of arc. Satisfactorywedge angles may be achieved, for example, with micrometer adjustments.Use of spherical mirrors eases somewhat the rigorous requirements forkeeping the still-amplifying beam in the cavity because sphericalsurfaces of the proper radius tend to reflect off-axis beams toward thecenter of the cavity. The laser radiation which emerges from a sphericalmirror has spherical wave fronts and thus is divergent. However, sincesuch coherent wave fronts appear to originate from a common center, theycan be, by use of a lens, made plane-parallel and hence, except fordiffraction effects, non-divergent.

By way of example, a continuously operating gas laser is disclosed in anarticle, Population Inversion and Continuous Optical Maser Oscillationin a Gas Discharge Containing He-Ne Mixture, Physical Review Letter, 6,page 106, 1961. In the usual embodiment of static gas, prior art gaslasers, the gas is statically contained in a tube about 100 centimeterslong. The mirrors which form the ends of the optical cavity are disposedeither inside the tube or external to it. Pumping is accomplished inthis system by electrical excitation (either radio frequency or directcurrent).

In addition to the helium-neon gas laser system, other gas lasersystems, have been achieved with helium, neon, argon, krypton, xenon,oxygen, and cesium (the last optically pumped in the gaseous state) asemitting atoms.

Other systems include carbon dioxide, helium, and nitrogen. For a morecomplete discussion of the highpower flowing system including carbondioxide, helium, and nitrogen reference is made to patent application ofC.K.N. Patel, Ser. No. 495,844, filed Oct. 14, 1965 and now abandonedand assigned to Bell Telephone Laboratories, Inc. Such a high-powerlaser typically includes two reflectors forming -a suitable resonator orcavity, a tube forming the sidewalls of the laser, suitable pumpingapparatus including a cathode, anode and direct-current sourcesconnected in appropriate polarity between the anode and the cathode;inlet apparatus; a source of carbon dioxide, helium, and nitrogenconnected to the inlet apparatus; and equipment for exhausting the spentgases from the laser or for cooling and separating them for reuse.

As indicated hereinabove, a laser output may be generated in variousmedia (i.e., crystals, semiconductors and gases) by pumping orintroducing energy to create an inversion where a large number of theatoms are in high energy levels to support photon emission. In prior artgas lasers, whether flowing or static, the lasers were pumped or excitedby using a diffusion controlled electrical discharge in a small tubemaintained at a low pressure. Typically, in such gas discharge tubes(typically of the order of one centimeter in diameter) operating at lowpressures (about 1-10 torr) there is a loss of electron-ion pairs fromthe center of the plasma to the sidewalls of the tube by radialdiffusion (socalled ambipolar diffusion of ion-electron pairs). For asteady state operation of the discharge, this loss must be made up by anet ionization rate in the plasma which exactly balances the diffusionloss rate. This required ionization rate dictates what temperature theelectrons must have to sustain the discharge, and hence what applied E/Nis needed to give the electrons that temperature. For long tubes E/N isdefined by the applied voltage divided by the tube length and gasdensity.

in such situations the discharge can be said to be ballasted" by thetube walls, i.e., since radial diffusion of the electron-ion pairs isfast, any small local increase in electron density is reduced bydiffusion. This fact makes such discharges radially and axially uniformas well as quite reliable and simple to produce.-

The plasma (neutral gas plus electron-ion pairs) contained inside theelectric discharge tube tends to remain radially uniform as long as thetime required for the electron-ion pairs to diffuse to the surroundingwalls is equal to the ionization time such as, for example, the timerequired to double the electron density. Since the ambipolar diffusiontime is generally proportional to the product of the gas pressure andthe tube diameter squared for large diameters, this ambipolar diffusiontime can, under some circumstances, become long compared to theionization time in the tube, especially for high ionization rates, largediameter tubes and high pressures. in this latter situation, thedischarge is no longer ballasted by the presence of the tube walls,i.e., local increases in the electron density are not immediatelydiffused to the walls where they are reduced by wall recombination, etc.Accordingly, local nonuniformities can be produced by these higherelectron densities and the fast-growing pon-uniformities can diffusionto the confining walls. Upon occurrence of such disturbances one canreduce their tendency to grow by reducing the ionization-rate whichmeans a lower electron temperature since the local ionization rate is afunction of the local electron temperature. A lower electrontemperature, however, requires that a lower electric field must beapplied. The proper balance is a critical one: too high an electricfield can allow the high pressure large diameter discharge to spoke, butif too. low an electric field is applied, the discharge cannot bestarted in the first place. Further, at high pressures, it is generallyfound that an applied voltage or electric field large enough to start adischarge is also large enough to cause the discharge to be radiallynon-uniform and, for example, spoke.

The present invention is directed to the production of and apparatus forproviding spatially uniform useful discharges in flowing gas lasers atpressure levels and sizes such that electron-ion pair diffusion to theconfining walls is negligible, that is, the discharge is not walldominated and is a true volume discharge.

It must be emphasized, however, the method and apparatus used in theproduction of said volumetric discharge is not limited to flowing gaslasers as noted below. Such-volumetrically scalable spatially uniformdischarges find uses in many other types of devices which use plasma forpropulsion, pumping, power generation, chemical processing, and thelike.

In its preferred embodiment, the present invention comprises a highpower flowing gas laser which is truly volumetric in character and thatcan be scaled in all three characteristic dimensions aswell as inpressure level. A spatially uniform discharge is created whereelectron-ion diffusion to the walls is truly negligible.

While the preferred embodiment of the present invention will bedescribed in connection with a flowing 2. Bridges and Patel: High PowerBrewster Window Laser at 10.6 Microns Appl. Phys. Lett. 7, 244 (1965).

3. Shapiro: Dynamics and Thermodynamics of Compressible Fluid Flow,Vol. 1. Ronald Press, N. Y., 1953.

4. Engelhardt, Phelps and Risk: Determination of Momentum Transfer andInelastic Collision Cross Sections for Electrons in Nitrogen UsingTransport Coefficients, Phys. Rev. 135, No. 6A, September 1964, p.A1566.

5. Phelps: Rotational and Vibrational Excitation of Molecules by LowEnergy Electrons, Westinghouse Research Laboratories, Scientific Paper67-1E2- GASES-P2, 1967.

6. Cobine: Gaseous Conductors, Dover Publications, N. Y., 1958.

7. Frost and Phelps: Momentum-Transfer Cross- Sections for SlowElectrons in He, Ar, Kr and Xe from Transport Coefficients, Phys. Rev.Vol. 136, No. 6A, December 1964.

8. Engelhardt, Phelps and Risk: Determination of Momentum Transfer andInelastic Collision Cross Sections for Electrons in N, using TransportCoefficients, Phys. Rev. Vol. 135, No. 6A, September 1964.

9. Hake and Phelps: Momentum-Transfer and Inelastic-Collision CrossSections for Electrons in 0 CO and CO Phys. Rev. Vol. l58,No. 1,June1969.

10. Cheo, Effects of GasFlow on Gainof 10.6

. Micron CO Laser Amplifiers, Journal of Quantum electrically excitednitrogen (N carbon dioxide (CO and helium (He) laser it may be appliedto other systems where such a plasma is required, including, but notrestricted to, lasers with gas constituents other than nitrogen, carbondioxide and helium as well as other lasing systems. A discharge inaccordance with the invention has the correct electron temperature formost efficient laser operation. Moreover, a laser in accordance with theinvention is volumetric in the sense that the proper gas temperatureand'lower laser state concentrations are maintained not by diffusionthrough the gas to cooled sidewalls, but rather by the proper choice ofgas flow velocity.

The following references and materials cited herein describe some of thebackground and physical principles involved in the present invention andan insight, to some degree, of the application of those principles inthe present state of the art:

1. Von Engel: Ionized Gases Oxford University Press, London, 1955.

Electronics,-Vol. QE-3,-No. 12, December 1967.

11. Guentherschulze, Der Kathodenfall der Glimmentladung inAbhaengigkeit von der Stromdichte Bei Spannungen Bis 3000 volts, Zeit,f. Physik, 38, p. 575, 1956.

l2. Schonhuber: Breakdown Below (Pcl),,,,,,, Proceedings of the 7thConference on Phenomena in Ionized Gases, Beograde, Jugoslavia, 1965,Beograd, Gradevinska Knjiga, 1966.

It is an object of the invention to provide apparatus for and a methodofproducing spatially uniform discharges in a gaseous medium.

It is another object of the invention to provide a spatially uniformdischarge in a gaseous medium in a controlled manner with small effecton background temperature, density and pressure of the medium.

A still further object of the invention is to provide apparatus for anda method of producing controlled, large, volumetric discharges withoutthe inherent ionization instability that occurs when the dischargecurrent itself produces the ionization.

A further object of the invention is to provide apparatus for and amethod of producing spatially uniform discharges in a gaseous mediumthat can be used, for example, to provide a lasing medium and otherapplications where a conducting gaseous medium is necessary or useful toachieve a desired result.

It is another object of the present invention to provide apparatus forand a method of producing a population inversion suitable for use in agas laser oscillator or amplifier.

It is another object of the present invention to provide apparatus forand a method of producing laser ac tion in a flowing gas by electricalexcitation.

It is a further object of the present invention to provide a gaseouslaser that is volumetrically scalable.

method of and apparatus for controlling the gas temperature in a gaseouslaser by proper choice of gas flow velocity and input power to increasethe efficiency of the lasing of the gaseous laser.

A still further object of the present invention is to provide a methodof an apparatus for producing laser action in a flowing gas bygenerating free electrons, and an electrical discharge to maintain theoptimum electron environment to produce the lasing action.

A still further object of the present invention is to provide a methodof an apparatus for producing laser action in a flowing gas byelectrical excitation comprising a short high voltage pulse to createelectrons and a DC discharge to maintain the optimum electronenvironment to produce lasing action.

A still further object of the invention is to provide an electricallyexcited flowing gas laser'wherein the arrangement of the electricalexcitation means results in optimum optical qualities.

7 The novel features that are considered characteristic FIG. 1; j

FIG. 3 is a sectional end view taken on lines 3-3 of FIG. 1;

FIG. 4 is a plot of electron temperature as a function of E/N for amixture-of pure helium, carbon dioxide,

nitrogen, as well-as a typical gas mixture used in electric N /COJHegaslasers;

FIG. 5 is a plot of rate constant as a function of electron temperaturefor upper and lower laser levels of CO, and vibrationally excited N,;and

FIGS. 6' A, B and C are plots showing the effect of gas temperature forvarious ratios of volumetric free electron density n in the gas to thepressure P of the gas required to produce high power and efficientvolumetrically scalable flowing gas lasers.

Attention is now directed to FIGS. 1-3 which illustrate a preferredembodiment of the invention. A gase ous medium capable of producinglasing action such as, for example, a mixture comprising 16% C0,, 34%N,and 50% He issupplied from a suitable source such as a plenum chamberand diffuser (not shown) to the working section of. the laser via gasinlet means 11. The working section of a laser in accordance with theinvention generally designated by the numeral 12 may be as showngenerally rectangular in configuration and com- I prise a frame 13 forremovably receiving oppositely disposed top and bottom sections 14and-l5 each adapted to receive a plurality of electrodes as shown andmore further described herein below. The working section 12 ispreferably comprised of an electrically nonconductive material as, forexample, Lucite,

Melamine, Fiberglass-Epoxy, and the like with the The top and bottomsections 14 and 15 may be made removable as shown to facilitateoperation and repair.

Sealably carried in the center portion of the top section 14 are aplurality of electrodes spaced one from another and hereinafter referredto collectively as sustainercircuit electrodes 21. The sustainer circuitelectrodes 21 extend through the top section 14 in the Y direction,through the space defined by the top and bottom sections and into thebottom section 15 such that their ends are recessed below the innersurface of the bottom section 15. Similarly, the bottom section issealably provided with a plurality of electrodes spaced one from anotherwhich are located respectively in the upstream and downstream portionsof the bottom section. These electrodes 22 and 23 extend through .theworking section and into grooves 24 and 25 defined by the frame 13 andtop section 14. The upstream electrodes are hereinafter collectivelydesignated by the pulser circuit electrodes 22 and the downstreamelectrodes are hereinafter collectively designated the common cathodeelectrodes 23. The tops of all of the electrodes are recessed in orderto insure uniform discharge in the active flowing gas medium bypreventing arcing from the tips of the electrodes. Disposed in the endwalls 26 and 27 of the frame 13 are respectively mirrors 28 and 29 whichdefine for the embodiment disclosed an optical cavity or lasing regionbetween the sustainer circuit electrodes 21 and the common cathodeelectrodes 23. Broadly, the lasing region is not limited to any onespecific location and may include a quite large region downstream ofelectrodes 22. The mirrors 28 and 29 are of conventional configurationand type adequate to define an optical cavity as are well known in theprior art. The cathode electrodes may be connected to ground throughisolation elements 31 such as resistors and/or capacitors.

Excitation and inversion of the gaseous medium in the region 35intermediate the sustainer electrodes 21 and common cathode electrodes23 is provided by a two-step process. Broadly, a high voltage dischargemay be provided between the pulser circuit electrodes 22 and commoncathode electrodes 23 by means of a conventional high voltage powersupply 36, pulser circuit 37 and'a trigger circuit 38. The pulsercircuit may comprise, by way of example, a: capacitor and aspark gaphaving a trigger electrode (not shown) triggered by a conventionaloscillator-type trigger circuit. The capacitor is charged by the highvoltage power supply and coupled to the pulser electrodes via the sparkgap. In order to continuously maintain optimum lasing conditions in thelasing region 35 a second discharge is providecl between the sustainerelectrodes 21 and the common cathode electrodes 23 by means of, forexample, a conventional DC power supply 45 and sustainer circuit 46comprising a plurality of capacitors each in series with a resistor.Altemately, an AC power supply may be used with appropriate circuitry.The capacitors and resistors referred to hereinabove may be replaced byresistors or inductors. The aforementioned capacitors and resistors areeach individually coupled to the power supply 45 and one of thesustainer electrodes in order to insure that each anode-cathode paircarries current through the gas about equally. In one successfulembodiment of the invention the working section 12 included 44 pulserelectrodes 22, 44 sustainer elec trodes 21 and 44 common cathodeelectrodes 23, as shown in FIG. 1, the spacing between the electrodes.being about 2.54 cm in the X direction and about 0.60 cm in the Zdirection. The upstream electrodes were about 0.05 cm in diameter. Inthis particular case the distance between the top and bottom sections 14and 15 was 2.54 cm and the distance in the Z direction between mirrors28 and 29 was about 30 cm.

The direction of laser output is perpendicular to the direction of flowof the gas and as indicated in drawing is in the Z direction, thedirection of gas flow being in the X direction, and preferrablesubstantially along the length of the longitudinal axis of the cavity.The optical cavity 35 is bounded by the sustainer electrodes 21 andcommon cathode electrodes 23 and the mirrors 28 and 29. In accordancewith conventional practice one of the mirrors such as, for example,mirror 29 is highly reflecting at the proper frequency and the other(mirror 28) is partially reflecting and partially transmissive to permitan output. As previously mentioned, the working gas passed through theworking section in the X direction as shown. The gas may be supplied ata pressure of, for example, 15 torr at a velocity of Mach 0.2. Asuitable pump (not shown) may be provided and coupled with the outlet ofthe working section to provide the desired pressure.

It is to be understood, however, that lasing action may be obtained evendownstream of electrodes 23, and that accordingly, excitation of theworking medium can be provided in a cavity or region separate from theworking or lasing region where the desired population inversion or laseraction is utilized.

As shown only by way of example in FIGS. 1-3, the present inventioncomprises broadly a flowing-gas laser which is truly volumetric incharacter and that is scalable in all three characteristic dimensions aswell as in pressure level. A spatially uniform discharge, more fullydescribed hereinafter is created in the working region whereinelectron-ion diffusion to the walls in the flowing gas is negligible. Itis to be understood that the present invention is useful with gasesother than that described in connection with the preferred embodiment, adischarge to effect pumping being provided to produce the correctelectron temperature for most efficient laser operation. Moreover, alaser, in accordance with the invention, is volumetric in the sense thatthe proper gas temperature and lower laser state concentrations aremaintained not by diffusion through the gas to cooled sidewalls, butrather, inter alia, by the proper choice of gas flow velocity.

Two problems found to be associated with producing a lasing medium whichis truly volumetric and which are overcome by the present invention arecontrol of gas temperature and discharge uniformity.

To facilitate an understanding of the invention, the problems first ofgas temperature control and then the provision of a uniform dischargewill now be discussed;

Directing attention now to gas temperature control consider firstconventional N -CO -He lasers pumped by means of a DC discharge. In theDC discharge-tube of such N -CO -He lasers, the gas therein is cooled byheat diffusion to typically water-cooled sidewalls and is kept uniformby the above as well as ambipolar diffusion of ion-electron pairs. Itcan be shown by a simple equation well known in the art that in such aDC steady-state situation, the energy balance for the gas in the tubetogether with the thermal conductivities of the gases used and the knownmaximum tolerable amount of gas heating allowed, limits the maximumamount of heat that can be conducted through the gas to cooled sidewallsto the order of 100-200 watts per meter of length of tube, depending tosome extent on the helium fraction used. Accordingly, since the maximumefficiency of the N -CO -He laser system is about 40 percent, theaforementioned heat loss of 100-200 watts per meter of length is alsoabout the maximum amount of laser power obtainable per meter from asteady state wall-cooled N -CO -He laser discharge tube. In actualtests, up to 75 watts of laser power have been obtained per meter ofdischarge tube in steady-state DC operation. The aforementioned limit ofsuch gas lasers is imposed by the requirement that the gas be cooled bythe walls of the confining tube. The axial (x) and radial (r)temperature distribution in a flowing thermally conducting gas is givenby the total energy equation pU C (ST /6x) l/r) (8/8r) r)t (ST/8r) Q (i)Where p the gas density, U the gas velocity, Cp the gas specific heat, Athe gas thermal conductivity, r the radial distance, x the axialdistance, Q the uniform volumetric heat generated, T the gastemperature, and T, =the stagnation temperature. The first or heatconvection term in equation (1) is the axial rise of gas temperature andis dependent on the gas pressure and flow velocity. The second or heatconduction term (the term to the left of the equal sign) is the heatconducted by the gas to the confining walls; and the third or heatsource term, Q, is a gas-pressure dependent volumetric heat source.

It may be seen from the equation (1) above that if the heat source term,Q, is balanced by the second or heat conduction term, an allowable tubediameter which is pressure-dependent is dictated to prevent over-heatingof the gas. However, if the heat source term, Q, is balanced by thefirst or heat convection term in equation (1), no such dependence ontube radius results. Moreover, since the heat source term and heatconvection term are both proportional to gas pressure, such pressurelevel effects tend to cancel out. That is, a higher pressure permitsmore energy input into the gas, but also results in a greater ability ofthe gas to store this energy before a temperature rise sets in. So faras bulk cooling alone is concerned, in the development of the presentinvention it was found that in order to have the temperature of the bulkof the gas controlled by flow rather than wall cooling, it is onlyrequired that the travel time of gas particles through the workingchamber of the laser be fast compared with the diffusion time of thatparticle from the center of the medium to the walls, i.e., the thermalboundary layers on the wall be thin compared to the height of thechannel.

For a parallel plate geometry, with a given plate spacing, gas velocity,and axial length the criterion for thin thermal boundary layers is thatthe time for thermal diffusion across the plate spacing be long com- (m/A) (H/2) L/U (2) where p the gas density (directly proportional topressure P), 0,, the gas specific heat (independent of pressure P), Athe gas thermal conductivity (independent of pressure), H the platespacing, U the gas flow velocity, and L the axial length of the plates.For typical values of gas pressure P equal to l torr of pure helium andan H of cm, a thermal diffusion time of about 1 second results. For achannel length, L, of about 100 cm and a flow velocity of Mach 0.1 (30meters per second) the travel time is 0.030 seconds. Accordingly, itwill now be seen that for the'case set forth, travel time will be muchshorter than the thermal diffusion time. thereby resulting in thinthermal boundary layers. With such thin thermal boundary layers thetemperature of the bulk of the gas will be flow controlled.

However, it was-also found that the provision of thin boundary layersper se referred to above was not sufficient for the choice of flowvelocity. As a result of a detailed study, it was found that there mustexist a match of gas flow velocity, gas pressure level, gas temperature,electron density, and average electron energy to provide a high power,efficient and volumetrically scalable flowing gas laser such as, forexample, the N /CO /He laser described hereinabove.

It is to be understood that while the technique now to be described wasdeveloped with a gas mixture of N /CO He, it is applicable to otherlasing systems and that other gases may be added if required or desired.The previous discussion about thin thermal boundary layers issignificant as the starting point fordetermining to'rise is mostimportant and will now be discussed in terms of the gain of the flowingN /CO /He laser described above.

The gain of any laser medium is in general proportional to thedifferencebetween the population of the upper and lower laser states(hereinafter designated respectively X and X of the system. If the upperlaser state is more highly populated than the lower state (X XL) thenthe gas or system is saidto possess a population inversion and. it can,in general, be made to produce laser action.

FIGS. 6 A, B and C show by way of example for a 3/2/1 mixture of He/N/CO the upper and lower laser state population X and X attained in anelectricallyexcited volumetric laser at a given volumetric electrondensity ri and pressure level P, low laser flux conditions for gastemperatures of respectively 300K, 500K and 700K. The electricaldischarge region and laser cavity were coincident for purposes of theseillustrative plots, although this is not in general required. Using theplots shown in FIG. 6 A, Band C it will now be shown that a proper gasvelocity must be chosen to prevent excessive gas temperature rise andhence inefficiency in the laser cavity with a given volumetric freeelectron density n.- at agiven pressure level P. For purposes of thisdiscussion it is assumed that the electrons are at or near the optimumelectrontemperature for laser excitation.

gas molecules (n /P) is not sufficiently high to provide a populationinversion. Accordingly X X and no lasing can occur. In region B theratio n /P is higher than in region A and population inversions arecreated (X X but the population of the upper laser state is notsubstantially greater than that of the lower laser,

state. Accordingly, only highly inefiicient lasing occurs. In region Cthe ratio n /P while moderate, is higher than that in region B and X XHere an efficient inversion is made, and useful lasing action occurs.The boundary between regions B and-C is where X is about one-fifth toone-half of X In region D the ratio n /P is higher than in region C butthe upper state population X is now sufficiently high that collisionaldeactivation of X produces a lower state population X, such that againthe net difference between X and X is reduced to about that existing inregion B. Accordingly, laser gain is reduced by at least a factor of 2from that existing in region C and only highly inefficient lasingactionoccurs. In region E no population inversion is obtained (as inregion A) because of the existance of an improper balance betweenexcitation rates and de-excitation rates.

1 From the preceding discussion and FIGS. 6 A, B and C it may be seenhow the proper choice of gas velocity in accordance with theinvention'can be made. Assuming that the incoming gas is at about-300K,'(see FIG. 6A) the volumetric free electron density n and mixturepressure level P are preferably chosen such that the gas or workingmedium starts into the laser cavity in region C. If, the power inputto-the gas (in this case electrical power) is, for example, at the low n/P end of region C,

a rise of about 400C in gas temperature cannot be tolerated since thepopulation inversion will be destroyed (see FIG. 6C). Accordingly, theflow velocity must be'increased, for example, to provide the necessarybalance of electrical power input and mass flow such that, for example,only a rise of l00K is achieved during the flow time through the cavity.If this is done, the working medium will be in region C at both thecavity inlet and outlet, and lasing in accordance with the inventionwill be provided. Alternatively, if the velocity is not changed, thenthe ratio n /P must be changed to maintain the-working medium in regionC. Whereas an inlet temperature of 300K has been used by way of example,it is to be clearly understood that the working medium may, if desired,be cooled and introduced at lower temperature with all of the advantagesinherent therein.

Increasing the gas velocity beyond this point decreases the gastemperature attained at the cavity exit and some small rise inefficiency may be obtained. However, if this is done anotherinefficiency now becomes important at a rate directly proportional tothe flow velocity. This further inefficiency results from the fact thatthe gas flowing out of the laser channel has been lasing, and hence isvibrationally excited. This flow of energy is a loss to the lasersystem, and is especially important in flowing lasers where, forexample, the ratio of flow area to mirror area is large. The preferredembodiment was operated generally in the range l0 n /P l0, which wasfound most efflcient for operation up to exit gas temperatures of about600K.

It will now be clear that increasing flow velocity from some minimalvalue will raise the volumetric laser overall efficiency in the cavityonly up to the point where the gas temperature at the laser exit ismaintained at a value such that the gain is substantially proportionalonly to the upper state population X there being only a small reductionin gain due to, for exam ple, the presence of a finite lower statepopulation X Further increases in flow velocity will reduce X towardzero, but, as previously pointed out, also results in an undesirableincrease in the flow of vibrationally excited gas out the exit of thecavity. In flowing lasers where this latter energy loss is anappreciable fraction of the laser output, the flow velocity required formaximum laser efficiency is reached when the increase in laserefficiency (by producing a smaller lower state population X resultingfrom an increased flow speed is just offset by the decrease in laserefficiency due to the resulting increase in convection out of the laserof the upper state population, X At this point the flow velocity isoptimized for laser efficiency in accordance with the invention byproviding the required match of electrical power input and gasmass-flow.

The preceding assumes that the pressure P has remained about constantand that the electric discharge producing the excitation has remained auniform discharge. Broadly, in the development of the invention it wasfound that spatially uniform discharges, as and for the purposes morefully described hereinafter, will produce efficient highpower laseroutput up to about the point where enough electrical energy has beenadded to the gas by the discharge'to roughly raise the initial gastemperature to some limit (about 600700I( for the embodiment describedherein) by Joule heating of the gas in the discharge in the timerequired for a discrete portion of the gas to flow through the lasersection (flow time), It should be noted that this gas temperature limitis compatible with the above-described'flow velocity/gas temperaturerelationship.

Beyond the aforementioned apparent limit formaximum obtainableefficiency the discharge was found to tend to become non-uniform andwhile lasing action does not cease the process becomes less efficient.However, this limit, as noted above, goes up with increasing pressureand gas velocity. Thus, in accordance with the invention as thepressure, flow velocity and scalesize are increased more and morepowermay be added to the gas while still providing a uniform dischargeand Mach number and it can be easily shown that the pump powerrequirements for a closed cycle system indicate that a low Mach-numberis, desirable. Thus, for a Mach number equal to 0.2 the pump powerrequired to overcome the pressure loss is approximately only one-fifththe laser output obtained from an N2'CO2-He cavity, and while at Mach2.0 the required pump power is at least 20 times this laser output. Foropen cycle operation, however, high speed flows are not inherentlydisadvantageous. Therefore, high speed flows (in excess of Mach 1.0 foropen cycle system) can be advantageous and allow the laser cavitypressure to be. less than one atmosphere and also have the workingmedium exhaust directly to the atmosphere.

In the experiments conducted in-the development of the present inventionit was found that a Mach number range of 0.05 to 0.6 is adequate forefficient laser operation.

Attention is now directed to the necessity of providing a uniformdischarge. As has been shown above, the use of flow for gas temperaturecontrol within the limits noted above allows dimensional scalingindependent of pressure level with the concurrent advantage that in adevice of fixed dimension using a suitable flow velocity for gastemperature control, increasing the pressure level permits an-increasein the output power of the laser. However, at such high pressures thedischarge must remain uniform and not change to arcs, streamers, and/orspokes. Moreover, the discharge at such high pressure must be able tosustain, without spoking, the electric fields required for laseroperation.

Data exists in the literature as to the average electron energy (hereincalled electron temperature) attained in electric discharges in singlespecies gases such as, for example, pure N CO He and the like. As wasdone in the development of thepresent invention, this data may be usedto predict the electron temperature in a gas discharge containing, forexample, a mixture of single .species gases. This procedure should usethe idea described above as to energy gain and loss for an electron-atomcollision in an applied electric field. Thus, given the data of electrontemperature, T,, and drift velocity, W vs E/N for each gas in questioncalculation can be made of T and W vs E/N for any mixture of thesegases. The electron temperature for pure N CO He, and a mixture of 16%CO 34% N and 50% He obtained in accordance with the above teaching isshown in FIG. 4. As may now be evident substantially any gas orcombination of gases such as CO, NO, H AR, N0 N 0 and the like may behandled in the manner discussed hereinabove and other gases may be addedif required or desired.

Using as a starting-point the literature on sealed-offdiffusion-dominated N -Co lasers, it was determined that a ratio of N toCO pressures of about 2:1 was optimum for power output and generallythat more helium than either N or CO, should be used. A mixture ofbetween l0 percent and 50 percent helium with the remaining gas beingtwo parts N to one part CO was found satisfactory.

The cross sections for direct excitation of the upper and lower laserlevels of CO by electron impact have been published along with those forvibrational excitation of N These cross sections converted to excitationrates are shown in FIG. 5. FIG. 5 shows that electron temperatures ofthe order of 0.80 to 1.50 electron volts is optimum for directexcitation of the upper laser level of CO (the 001 state) and the firstvibrational level of N (V=l) which efficiently transfers its energy tothe C0 001 state.

For the above mixture an E/N of about 1-3 X 10" volts per centimeter perparticle per cubic centimeter yields an electron temperature of about 1eV.

From the above it may now be seen that for optimum operation of the N-CO system, for example, the preferred region of operation is one wherethe rate of energy loss from the electrons in the discharge isdominately due to vibrational excitation of the N and CO and that,accordingly, the electron temperature should be fixed at about 1 eV.This requirement, however, is at least insome cases incompatible withthe requirement that in a DC diffusion dominated laser where for reasonsof discharge uniformity the production rate of electrons must exactlybalance the net rate of electron loss. Where the latter situation is thecase the electron temperature is tied to the pressure level and tubesize and is not necessarily l 'eV.

The provision of the two discharge pumping techniques used in thepreferred embodiment circumvents the above limitation by essentiallyapplying two successive discharges to the gas as it flows through thechannel.

Broadly, in the preferred embodiment the first discharge creates theelectron density uniformly using only. a small amount of energy whilethe second discharge provides a voltage to give these electrons atemperature sufficiently high for laser action but not high enough togenerate any appreciable increase in electron density. The vsecond orsustainer discharge puts the dominant amount of energy into the gasdirectly where it is desired. In the case of the N -CO laser-the energyis put into the upper laser'state of CO, and into Nitrogen vibration,the optimum electron temperature assuring optimum laser efficiency. Uponcreation by the first discharge of a uniform electron-ion cloud, thecloud stays uniform during the time of the second discharge as long asthe second discharge does tion. If the level of the second or sustainerdischarge is raised to the point where it too producesa rapidionization, then discharge nonuniformities may be created. However,provision of a second or sustainerv discharge selected to slowly createelectrons results in maintenance of stable uniform discharge for severalflow times. This slow creation of electrons by the second dischargenearly balances the loss of electrons produced by the first dischargedue to flow and recombination electron losses and must not substantiallyincreasethe level of ionization as this will result in a nonuniformelectron density such as arcing.

As will not be apparent, the present invention permits the provision ina flowing gas laser of a spatially uniform discharge at the optimumelectron temperature required for efficient laser operation at arbitrarypressure levels and physical sizes. While the invention is not solimited,;this may be accomplished byutilization of the aforementionedtwo-step discharge comprising, preferably, first a fast high-voltagedischarge which creates a uniform electron density which wouldordinarily, if left on its own, disappear by volumetric processes whileflowing out of the channel and be incapable of producing efficient highpower laser action. However, a second lower voltage discharge isprovided. which gives the electrons produced by the first discharge thenecessary electron temperature for preferably optimum laser excitation,with no significant increase in electron density.

It is to be understood that the invention is not limited to theapparatus shown and described and that, for example, other methods ofand apparatus for creating the initial electron density can be used suchas ultraviolet radiation, a high energy beam of electrons, protons andthe like provided by electron beam means for introducing one or moreelectron beams to produce ionization of the gaseous medium as and forthe purposes set forth hereinabove. Irrespective of whether theelectrons are generated in the above described manner or any othersuitable manner, they must be heated to the correct electron temperatureby the E/N applied by the sustainer discharge.

Attention is again directed to FIGS. 1-3 which are illustrative of thepreferred embodiment actually reduced to practice and which incorporatedthe previously described two-step pulser-sustainer discharge in agaseous mixture of N CO and He.

Because of the rather high voltages that may be required for the pulsercircuit shown and described for producing a first discharge, sufficientvoltage standoff distances must be provided both upstream and downstreamof the electrodes to assure current flow in the gas only in the properdirection in the pulser circuit. In lasers, in accordance with theinvention, this may be accomplished, as previously pointed out, by, forexample, fabricating the working section itself of strong, electricallynonconductive material such as, for example, Melamine or fiberglassimpregnated with an epoxy. Since lasers in accordance with the presentinvention are scalable, large channels, for structural reasons, may

require metallic walls. In this case the internal surfaces in at leastthe area of the discharge must be sheathed with-a suitable nonconductingmaterial such as, for example, quartz and the like.

Where metallic mirrors are used it is important that they not be locatedtoo close to the discharge area as the electrodes adjacent to themirrors will tendto discharge into them rather than through the flowinggas as desired. Accordingly, a sufficient voltage standoff distance ofthe mirrors from the discharge is required to prevent an undesireddischarge tothe mirrors. A

separation of the mirrors from the electrodes of about dicular to a highspeed gas flow is not directly across the anode-to-cathode gap but thecurrent path is blown downstream and the higher the gas velocity thegreater the current path distortion. As will now be obvious,

I with the current paths chosen parallel to the gas flow in accordancewith the invention, this problem is not encountered.

If the electrode axes are chosen, for example, in the Z direction eventhough current flow in the X direction chosen so that theambipolar'diffusion in the Z direction is just sufficient to causeadjacent current sheets to merge, even at high pressures and high flowvelocities so that all of the gas flowing through the working regionundergoes the excitation process and is thereby made available forlasing. For any given case, a

suitable transverse (Z-direction) spacing may be calculated by use ofthe ambipolar diffusion time, the anodecathode spacing, and electrodediameters.

Cathode electrodes have a greater voltage drop during operation andhence they provide correspondingly greater heating than anodeelectrodes. For this reason the cathode electrodes should be placed atabout the exit of the laser cavity downstream of the anode electrodes.Such an arrangement has the advantage of providing a cooler gastemperature for the lasing medium in the working region and hence a moreefficient laserprocess. In actual tests, discharges that would justmarginally produce laser action with the embodiment shown in FIG. 1 werefound not'toproduce laseraction at all when the cathodes were locatedupstream of the nonuniform discharge'and consequent poor lasingqualities.

It is to be noted that the configuration of the electrodes is notlimited to cylindrical electrodes and that other configurations may beused. For example an airfoil shape with polished surfaces (not shown)may be used. An air-foil configuration has the advantage of reducing thepressure drop due vto the aerodynamic drag of the electrodes. Further,the electrodes should be spaced as closely together as possible as thiswill tend to result in the entire electrode assembly acting as a hollowcathode, thereby permitting the ultraviolet radiation inherent therewithto be used to augment electron removal from the metal surfaces over thatresulting from the cathode voltage drop. Still further, the largeremitting surfaces of air-foil electrodes provide the additionaladvantage of helping current emission in terms of lower cathode voltagedrop requirements. As to electrode materials, suitable electrodes as itsenergy source. Insurance that each individual electrode pair will carrycurrent may also be accomplished by heavily ballasting each suchanode-cathode pair with high resistance elements and driving all of theelectrodes with a'single large capacitor or even directly 7 i from thepower supply. However, this latter approach is generally wasteful ofpower due to the ohmic dissipation in the large ballast resistors thatare required. Both the above methods may be used, as might other circuitconfigurations and elements, as, for example, induction elements inplace of or in addition to, resistor/capacitor combinations the choiceof which will be dictated by the specific application. The capacitorsfor the sustainer anode-cathode circuits should be of such a size thatthey will not be appreciably discharged if the sustainer circuitoperates on a pulse-basis] A decrease of 5 to 10 percent of the initialvoltage hasbeen found satisfactory. The sustainer power supply, whichmay be of conventional configuration, must be of sufficient capacity torecharge the sustainer capacitors before the next pulse of current isrequired from the capacitors. Accordingly, the required chargingtimewill be determined by the, repetition rate required of the dischargepulses. This in turn depends on the use to which a laser in accordancewith the invention is to be put. High repetition pulse rates produce anessentially DC laser output while low repetition rates, as may berequired in some instances, produce a pulsed output. A sustainerdischarge of the order of milliseconds in duration has been foundsatisfactory, hence the inductance of the circuit gives rise to verysmall voltage drops. The voltage that need be applied across thesustainer capacitors is determined by the E/N considerations mentionedpreviously herein as well as the cathode and anode drops measured forthe given electrode geometry, material, surface condition, currentdensity and pressure level.

Accordingly, while the pressure level, gas constituents, electrodespacing and electron temperature requirements give the required voltagedrop across the positive column of the discharge, the actual voltageapplied to the capacitors in the sustainer circuit may be some hundredsof volts higher due to electrode drops. If the sealing of the laser andoperating conditions are such that kilovolts are determined to benecessary for laser action then, of course, hundred-volt electrode dropswill be unimportant. If, however, it is determined that only hundreds ofvolts are required across the positive column, then some amount ofempiricism may be required to provide the most suitable capacitorvoltage.

The pulser circuit comprises the aforementioned pulser anodes andcathode electrodes. A conventional high voltage power supply is coupledto a charging circuit coupled to the pulser circuit anodes via a triggercircuit. The pulser circuit power supply may comprise a conventional DCvoltage supply capable of providing 1-20 kV or more. The chargingcircuit is coupled to the power supply and may comprise a capacitor inseries with a limiting resistor (not shown). The trigger circuit maycomprise a conventional spark gap (not shown) with a trigger electrodecoupled to' a conventional oscillator or the like adapted. to supply thedesired pulse voltage, pulse length and repetition rate. The shape ofthe fast high-voltage pulse (i.e., rise-time, decay time, and voltageand current magnitude) was found important for laser efficiency in someoperating regimes, pulse shape was influenced by the inclusion ofpulse-shaping resistors (not shown) inthe pulser circuit. Suitablevalues of these resistors vary with, for example, pressure level in thelaser channel and pulsercircuit (energy storage) capacitance.

As the working gas enters the working region at Mach numbers of, forexample, 0.05-0.60 and presdownstream of the pulser anode electrodes)that the 7 required. initial spatially uniform electron density iscreated. The electrons so produced together with the balance of theplasma created by the pulser circuit discharge then passes through thesustainer anode electrodes where the electrons are heated to theelectron temperature required for the gas being used such that, for thecase of the N -CO -He gas mixture for example, the upper laser levels ofN and CO are preferentially excited by electron impact for maximum laserefficiency. For a laser or oscillator the laser energy may be extractedin the conventional manner by the mirrors disposed at each end of thecavity bounded by the sustainer and ground electrodes and the sidewallsof 25 the frame. For an amplifier, the mirrors may be replaced bysuitable windows to permit the required light beam to pass through thecavity and be amplified in conventional manner.

While the voltage of the sustainercircuit has been successfully operatedat voltages of the order of 1000 volts or less and may. be appliedcontinuously, the pulser circuit voltage has been successfully appliedwith no lack of uniformity in the discharge at the rate of, for example,100 pulses/second with a pulse width of from 20-100 nanoseconds, up to arise time of as long as one microsecond and a decay time of severalmicroseconds. Satisfactory operation has been obtained with the pulsercircuit voltage being applied as little as once every two or three flowtimes, i.e., the time it takes for the gas to flow from the sustainercircuit anodes to the grounded cathode electrodes. Time scales of theorder noted above may be expected with other gases such as, for example,CO NO, N 0, N S0,, HCl, HBr, HI, HF, Ar, and the like.

In tests it was found that for the N -CO -He laser referred tohereinabove, with a single pulser circuit capacitor of 0.003microfarads, optimum lasing occurred as pulser voltages (across a 2 inchgap in the X direction) of kV for a pressure of 15 and 30 torr and at apulser voltage of kV for 45 torr. Further, the pulser circuit voltagerequired depends, at least in part, on the size of the gap. It was alsofound that increasing the pulser circuit capacitor by a factor of 7 andhence increasing the pulser circuit energy input by a factor of 7increased the sustainer circuit energy input and hence laser output byonly a factor of 2. At such a point the entire system is becoming lessefficient because the pulser energy input approaches the sustainercircuit input. it was further found that in the initial part of thesustainer circuit pulse, high currents were present due to the highinitial electron density produced by the pulser circuit. As theseelectrons disappear by volume recombination the sustainer circuitcurrent falls and finally the sustainer discharge quenches. Laserradiation showed an initial period of about 10-30 microseconds where nolasing action took place. Then, a threshold appeared to be crossed andthe laser turned on. At this point the laser output time history closelyfollowed the current input time history.

Exemplary operating parameters for. an embodiment actually reduced topractice are set forth in Table 1 below.

Laser Positive Output Wavelength Output Coupling Peak Pulsed OutputPower Repetition Rate Pulse Width (Pulser Circuit) 1 Pulse Width.(Sustainer Circuit) Gas Input Pressure lnput Velocity Laser Cavity SizeElectrodes Pulser Circuit: High Voltage Power Supply Charging ResistorsPulse Shaping Resistors Energy Storage Capacitor Pressurized Spark GapTrigger Circuit Sustainer Circuit Power Supply Resistors CapacitorsBallast Resistor for Each Electrode Pair TABLEI 50 to 1000 Watts 1 topps 20 to I00 nanoseconds (to several microseconds) To 6 Milliseconds16% C0,, 34% N 50% He 15 torr (15 to 45 torr) 0.2 Mach (0.05 to 0.6Mach) 2.54 cm wide X2.54 cm high X 30 cm long 44 tungsten pulser circuitanodes 4'4 tungsten sustainer circuit anodes 44 tungsten common cathodes20Kv at l. inilliamp 50K to 500K ohms 720 ohms 0.003 to 0.030 pfd 0.60cm gap width l-l00 pps 3Kv at 6 amps 20K ohms 500 ohms The variousfeatures and advantages of the invention are thought to be clear fromthe foregoing description. Various other features and advantages notspecifically enumerated will undoubtedly occur to those versed in theart as likewise will many variations and modifications of the preferredembodiment illustrated, all of which may be achieved without departingfrom the spirit and scope of the invention defined by the followingclaims.

lclaim:

1. In the method of producing a spatially uniform dischargesubstantially throughout a gaseous working medium in a working region,the steps comprising:

a. providing a gaseous working medium at a pressure in a working regionthat upon the production of free electrons in said medium said mediumhas ambipolar and thermal diffusion rates incapable of substantiallydamping local increases in said free electron density in said medium;

b. generating substantially throughout said working region asubstantially spatially uniform density of free electrons in said mediumby ionizing said medium; and

c. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of saidfree electrons withoutsubstantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing saiddischarge substantially uniformly throughout said working region at apredetermined level.

2. The method as defined in claim I and additionally includingmaintaining the level and uniformity of both the density and temperatureof said medium at values less than that which will produce substantialarcing in said medium.

3. In the method of producing a spatially uniform dischargesubstantially throughout a gaseous working medium in a working region,the steps comprising:

a. passing said gaseous working medium at a pressure through a workingregion that upon the production of free electrons in said medium saidmedium has ambipolar and thermal diffusion rates incapable ofsubstantially damping local increases in secondary electron density insaid medium;

b. generating substantially throughout said working region asubstantially spatially uniform density of free electrons in said mediumby ionizing said medium;

0. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said free electrons withoutsubstantially increasing said predetermined electron density byself-regenerative ionization; and

. maintaining the average energy of said free electrons and the leveland uniformity of both the density and temperature of said medium atvalues to produce said spatially uniform discharge in said medium.

4. The method as defined in claim 3 wherein said free electrons aregenerated with an average energy insufficient to produceselflregenerative ionization but with a density sufficient to supportsaid discharge.

5. The method as defined in claim 4 wherein the average energy of saidfree electrons is less than that which will produce a non-uniformdischarge.

6. The method as defined in claim 5 .wherein said working region is in acavity and the level and uniformity of the density and temperature ofsaid medium are respectively maintained at values that for an averageenergy of said free electrons less than that which will produce anon-uniform discharge, produce said spatially uniform discharge in saidmedium during flow through said cavity.

7. In apparatus for producing a spatially uniform dischargesubstantially throughout a gaseous working medium in a working region,the combination comprismg:

a. gas supply means for producing a flow of a gaseous working mediumhaving a predetermined velocity and pressure;

. means defining a working region for receiving said medium from saidgas supply means and through which said flow passes, said working regiondefining a predetermined cross section and volume that at said pressuresaid gaseous working medium in said working region has ambipolar andthermal diffusion rates incapable of substantially damping localincreases in electron density in said medium;

c. first means for producing substantially throughout said workingregion a substantially spatially uniform density of free electrons insaid medium by ionizing said medium; and

d. second means for providing a sustainer field for controlling theelectron temperature of said free electrons in said medium tosubstantially uniformly throughout said working region increase theiraverage energy without substantially increasingthe density thereof byself-regenerative ionization at said velocity and pressure and producesaid discharge substantially uniformly throughout said working region.

8,. The combination as defined in claim 7 wherein said first meansproduces said free electrons with an average energy insufficient toproduce self-regenerative ionization but with a density sufficient tosupport said discharge; and said second means includes means forproducing an electric field in said working region.

9. The combination as defined in claim 7 wherein said first and second.means serially provide said discharge in the form of pulses, said mediumhaving a flow time through said working region that is long compared tothe time of each of said pulses.

10. The combination as defined in claim 9 wherein said working region isin a cavity said meansdefining said cavity includes gas inlet and gasoutlet means and said'second means includes electrode means disposedintermediate said gas inlet and outlet means for producing asubstantially uniform electric field substantially throughout saidworking region.

11. In the method of light generation by stimulated emission ofradiation in a working region having a longitudinal axis .the stepscomprising:

a. providing a gaseous working medium having an upper and a lower laserstate in said working region;

b. providing a substantially spatially uniform predetermined density offree electrons in said medium substantially throughout said workingregion;

c. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said free electrons withoutsubstantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing anaverage energy level sufficient to support a population inversion insaid medium; and

d. providing a pressure of said medium in said working region to producea population inversion in said medium in said working region.

12. The method as defined in claim 11 wherein said electron density andgas pressure and flow velocity through said working region are selectedto produce substantially maximum population inversion in said medium insaid working region.

13. The method as defined in claim 11 wherein said working region is ina cavity. 2

14. The method as defined in claim 13 wherein said gaseous workingmedium is a mixture of helium, nitrogen and carbon dioxide.

15. The method as defined in claim 14 wherein said medium comprisingsaid mixture is introduced into said cavity at about 300 Kelvin and isexhausted from said cavity at about 600 Kelvin.

16. In the method of light generation by stimulated emission ofradiation in a'working region in a cavity having a gas inlet, a gasoutlet and a longitudinalaxis the steps comprising:

a. passing a gaseous working medium having an upper'and a lower laserstate through said working region in a direction orthogonal to saidaxis;

b. providing a substantially spatially uniform predetermined density offree electrons in said medium substantially through said working region;

. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said free electron withoutsubstantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing anaverage energy levelsufficient to support a population inversion in saidmedium; and i providing a velocity and pressure of said medium in saidworking region to produce a population inversion in said medium in saidworking region wherein any increase in population inversion due to adecrease in the population of said lower laser state resulting fromsubstantially any further increase in gas velocity is not substantiallyless than any decrease in population inversion resulting from theincrease in flow of said upper laser state out of said working region. 1i

17. In the method of light generation by stimulated emission ofradiation substantially throughout a gaseous active medium in a workingregion in a cavity having a gas inlet, a gas outlet and a longitudinalaxis the steps comprising:

a. providing a gaseous active medium at a pressure in a working regiondisposed in a cavity having walls for confining the gaseous workingmedium that upon the production of free electrons in said medium saidmedium has ambipolar and thermal diffusion rates incapable ofsubstantiallydamping local increases in free electron density in saidmedium, said medium having an upper and lower laser state;

b. generatingsubstantially throughoutsaid working region asubstantiallyspatially uniform predetermined density of free electrons in said mediumhaving an average energy insufficient to produce a population inversionin said mediumy'and providing a sustainer field for providingsubstantially uniformly throughoutsaid working region a predeterminedelectron temperature effective to increase the average energy of saidfree electrons without substantially increasing said predeterminedelectron density by self-regenerative ionization, said electrontemperature producing an average energy level sufficient to support apopulation inversion in said medium.

18. The method as defined in claim 17 wherein said medium is passedthrough said cavity orthogonal to said axis and said sustainer field isan electric field both orthogonal to said axis and anti-parallel to thedirection of flow of said medium.

19. The method as defined in claim 17 wherein said sustainer field is enelectric field both orthogonal to said axis and parallel to thedirectionof flow of said medium. 7

20. The method as defined in claim 17 wherein said free electrons areprovided by means of a discharge.

21. The method as defined inclaim 20 wherein said discharge is providedin the form of pulses whereby said spatially uniform density of freeelectrons is sequentially provided at discrete spaced intervals of time.

22. The method as defined in claim 21 wherein each of said pulses isprovidedfor a time that is short compared to the time required for saidgas to flow through said working region.

23."The method as defined in claim 21 whereineach of said pulses isprovided for a time of less than about l0 microseconds.

24. The method as defined in claim 17 wherein said working region has alongitudinal axis and said medium is passed through said working regionsubstantially uniformly all along and orthogonal to said axis; and saidsustainer field is an electric field orthogonal to said axis andparallel to the direction of flow of said medium.

25. The method as defined in claim 24 wherein said free electrons areprovided bymeans of an electrical discharge.

26. The method as defined in claim 25 wherein said discharge is seriallyprovided in the form of pulses and the energy added to the medium bysaid pulses is small compared to the energy added to the medium by saidelectric field.

27. The method as defined in claim 26 wherein each of said pulses isprovided for a time that is short compared to the time required for saidgas to flow through said working region and each of said discharges isprovided by a voltage that ishigh compared to that of said electricfield.

28. The method as defined in claim 27 whereineach of said pulses isprovidedfor a time of less than about ten microseconds.

29. Themethod as'defined in claim 24 wherein said medium is passedthrough said cavity at less than supersonic velo'city.

30. in high powered laser apparatus the combination comprising:

a. gas supply means for producing a flow of a gaseous medium having apredetermined velocity and pressure and an upper and lower laser state;

means defining a cavity including a working region for receiving saidmedium from said gas supply means and through'which said flow passes;

c. first means for producing a spatially-uniform density of freeelectrons in said medium substantially throughout said working region byionizing said medium, said free electrons having an average energyinsufficient to produce a population inversion in said'medium; and

d. second means for providing a sustainer field for controlling theelectron temperature of said free electrons in said medium tosubstantially uniformly throughout said working region increase theiraverage energy without substantially increasing the density thereof byself-regenerative ionization at said velocity and pressure and produce apopulation inversion in said medium in said working region.

31. In high powered laser apparatus the combination comprising: i I

a. gas supply means for producing a flow of an ionizable gaseous mediumhaving a predetermined,

velocity and pressure;

b. means defining an elongated optical cavity having a longitudinalaxis, said means receiving said medium from said gas supply means in adirection orthogonal to and extending along the length of said axis andthrough which said flow passes;

0. first means for ionizing said medium and produce a uniform density offree electrons in said medium, said electrons having an average energyinsufficient to produce a population inversion in said cavity; and

d. second means for producing an electric field in said mediumcontaining said free electrons to increase the average energy of saidelectrons produced by said first means without substantially increasingthe density of said electrons by selfregenerative ionization and producea population inversion in said medium during flow through said cavity atsaid velocity andpressure.

32. The combination as defined in claim 31 wherein said first meansincludes further means for providing a discharge in said medium for atime that is short compared to the time required for said medium to flowthrough said cavity.

33. The combination as defined in claim 32 wherein said discharge isprovided for a time less than about microseconds. v

34. The combination as defined in claim 32 wherein said further meansserially provides said discharge in the form of pulsed and adds energyto said medium in an amount that is small compared to that of saidsecond means.

35. The combination as defined in claim 34 wherein said second meansfurther includes means for providing the majority of the energy input tosaid medium.

36. The combination as defined in claim 35 wherein said means definingsaid cavity includes gas inlet and gas outlet means and said secondmeans includes first and second electrode means disposed intermediatesaid gas inlet and outlet means, said longitudinal axis being disposedintermediate said first and second electrode means.

. 37. The combination as defined in claim 36 wherein said first meansincludes third electrode means disposed upstream from said axis.

38. The combination as defined in claim 37 wherein said second electrodemeans are cathodes electrodes common to said first and second means andare disposed downstream of said first electrode means.

39. The combination as defined in claim 37 wherein said first, secondand third electrode means comprise rods spaced one from anotherextending across said cavity orthogonal to said longitudinal axis anddisposed along the length of said axis.

40. The combination as defined in claim 35 wherein said cavity includesend walls having means for passing a light beam through said cavityparallel to said axis and downstream of said first electrode.

41. In high powered laser apparatus the combination comprising:

a. gas supply means for producing a flow of an ionizable gaseous mediumhaving a predetermined velocity and pressure, said medium having anupper and a lower laser state; means defining an elongated cavity havinga lonitudinal axis, said means receiving said medium rom said gas supplymeans in a direction orthogonal to and extending along the length ofsaid axis and through which said flow passes; said cavity includingoppositely disposed end walls having means defining an optical cavityparallel to said axis;

c. first means for ionizing said medium and produce substantiallythroughout said optical cavity a uniform density of free electrons insaid medium;

and

(1. second means for producing substantially throughout said opticalcavity an electric field in said medium containing said free electronsto increase the average energy of said electrons without substantiallyincreasing the density of said electrons by self-regenerative ionizationand produce a population inversion in said medium during flow throughsaid optical cavity.

42. The combination as defined in claim 41 wherein said first meansincludes further means for providing a discharge in said medium for atime that is short compared to the time required for said medium to flowthrough said optical cavity.

43. The combination as defined in claim 42 wherein said discharge isprovided for a time less than about 10 microseconds.

44. The combination as defined in claim 42 wherein said further meansserially provides said discharge in the form of pulses and adds energyto saidmedium in an amount that is small compared to that of said secondmeans.

45. The combination as defined in claim 44 wherein said second meansfurther includes means for providing the majority of the energy input tosaid medium.

46. The combination as defined in claim 45 wherein said means definingsaid cavity includes gas inlet and gas outlet means and said secondmeans includes first and second electrode means disposed intermediatesaid gas inlet and outlet means, said longitudinal axis being disposedintermediate said first and second electrode means.

47. The combination as defined in claim 46 wherein said first meansincludes third electrode means disposed upstream from said axis.

48. The combination as defined in claim 47 wherein said second electrode'mearis are cathodes electrodes common to said first; and second inearisand are disposed downstream of said first electrode means.

49. The combination as defined in claim 47 wherein said first, secondand third electrode means comprise rods spaced one from anotherextending 'across said cavity orthogonal to said longitudinal axis anddisposed along the length of said axis.

50. The combination as defined in claim 46 wherein said means definingsaid optical cavity comprises means for passing a light beam throughsaid cavity parallel to said axis and downstream of said firstelectrodemeans.

" UNITED STATES PATENT OFFICE CERTIFIEATE ()F CORRECTION Patent No. 3,721, 915 Dat d March 20, 1973 Inventor(s) James P. Reilly It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

4 Column 8, line 21, for "tops", read--tips--; Column 10, line 21,

for "pUC T /x (l/r) 6/6r m (GT/6r) Q", read-- pUC 7824) (l/r)(8/8 r)r7l(8 T/8 r) Q-; Column 11, line 48, for "(X X read--(X 7X Column 18, line45, for "sealing", read- -scaling--; Colimn 23, claim 19, line 2, for"en read--an--; and Column 25, claim 34, I line 3, for "pulsed"read--pulses Signed and sealed this 16th day of December 1973.

(SEAL) Attest:

EDWARD M. FLETCEER, JR RENE D TEGIWLMER Atte sting Officer ActingCommissioner of Patents

1. In the method of producing a spatially uniform dischargesubstantially throughout a gaseous working medium in a working region,the steps comprising: a. providing a gaseous working medium at apressure in a working region that upon the production of free electronsin said medium said medium has ambipolar and thermal diffusion ratesincapable of substantially damping local Increases in said free electrondensity in said medium; b. generating substantially throughout saidworking region a substantially spatially uniform density of freeelectrons in said medium by ionizing said medium; and c. providing asustainer field for providing substantially uniformly throughout saidworking region a predetermined electron temperature effective toincrease the average energy of said free electrons without substantiallyincreasing said predetermined electron density by self-regenerativeionization, said electron temperature producing said dischargesubstantially uniformly throughout said working region at apredetermined level.
 2. The method as defined in claim 1 andadditionally including maintaining the level and uniformity of both thedensity and temperature of said medium at values less than that whichwill produce substantial arcing in said medium.
 3. In the method ofproducing a spatially uniform discharge substantially throughout agaseous working medium in a working region, the steps comprising: a.passing said gaseous working medium at a pressure through a workingregion that upon the production of free electrons in said medium saidmedium has ambipolar and thermal diffusion rates incapable ofsubstantially damping local increases in secondary electron density insaid medium; b. generating substantially throughout said working regiona substantially spatially uniform density of free electrons in saidmedium by ionizing said medium; c. providing a sustainer field forproviding substantially uniformly throughout said working region apredetermined electron temperature effective to increase the averageenergy of said free electrons without substantially increasing saidpredetermined electron density by self-regenerative ionization; and d.maintaining the average energy of said free electrons and the level anduniformity of both the density and temperature of said medium at valuesto produce said spatially uniform discharge in said medium.
 4. Themethod as defined in claim 3 wherein said free electrons are generatedwith an average energy insufficient to produce self-regenerativeionization but with a density sufficient to support said discharge. 5.The method as defined in claim 4 wherein the average energy of said freeelectrons is less than that which will produce a non-uniform discharge.6. The method as defined in claim 5 wherein said working region is in acavity and the level and uniformity of the density and temperature ofsaid medium are respectively maintained at values that for an averageenergy of said free electrons less than that which will produce anon-uniform discharge, produce said spatially uniform discharge in saidmedium during flow through said cavity.
 7. In apparatus for producing aspatially uniform discharge substantially throughout a gaseous workingmedium in a working region, the combination comprising: a. gas supplymeans for producing a flow of a gaseous working medium having apredetermined velocity and pressure; b. means defining a working regionfor receiving said medium from said gas supply means and through whichsaid flow passes, said working region defining a predetermined crosssection and volume that at said pressure said gaseous working medium insaid working region has ambipolar and thermal diffusion rates incapableof substantially damping local increases in electron density in saidmedium; c. first means for producing substantially throughout saidworking region a substantially spatially uniform density of freeelectrons in said medium by ionizing said medium; and d. second meansfor providing a sustainer field for controlling the electron temperatureof said free electrons in said medium to substantially uniformlythroughout said working region increase their average energy withoutsubstantially increasing the density thereof by self-regenerativeionization at said velocity and pressure and produce said dischargesubstantially uniformly througHout said working region.
 8. Thecombination as defined in claim 7 wherein said first means produces saidfree electrons with an average energy insufficient to produceself-regenerative ionization but with a density sufficient to supportsaid discharge; and said second means includes means for producing anelectric field in said working region.
 9. The combination as defined inclaim 7 wherein said first and second means serially provide saiddischarge in the form of pulses, said medium having a flow time throughsaid working region that is long compared to the time of each of saidpulses.
 10. The combination as defined in claim 9 wherein said workingregion is in a cavity said meansdefining said cavity includes gas inletand gas outlet means and said second means includes electrode meansdisposed intermediate said gas inlet and outlet means for producing asubstantially uniform electric field substantially throughout saidworking region.
 11. In the method of light generation by stimulatedemission of radiation in a working region having a longitudinal axis thesteps comprising: a. providing a gaseous working medium having an upperand a lower laser state in said working region; b. providing asubstantially spatially uniform predetermined density of free electronsin said medium substantially throughout said working region; c.providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said free electrons withoutsubstantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing anaverage energy level sufficient to support a population inversion insaid medium; and d. providing a pressure of said medium in said workingregion to produce a population inversion in said medium in said workingregion.
 12. The method as defined in claim 11 wherein said electrondensity and gas pressure and flow velocity through said working regionare selected to produce substantially maximum population inversion insaid medium in said working region.
 13. The method as defined in claim11 wherein said working region is in a cavity.
 14. The method as definedin claim 13 wherein said gaseous working medium is a mixture of helium,nitrogen and carbon dioxide.
 15. The method as defined in claim 14wherein said medium comprising said mixture is introduced into saidcavity at about 300* Kelvin and is exhausted from said cavity at about600* Kelvin.
 16. In the method of light generation by stimulatedemission of radiation in a working region in a cavity having a gasinlet, a gas outlet and a longitudinal axis the steps comprising: a.passing a gaseous working medium having an upper and a lower laser statethrough said working region in a direction orthogonal to said axis; b.providing a substantially spatially uniform predetermined density offree electrons in said medium substantially through said working region;c. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said free electron withoutsubstantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing anaverage energy level sufficient to support a population inversion insaid medium; and d. providing a velocity and pressure of said medium insaid working region to produce a population inversion in said medium insaid working region wherein any increase in population inversion due toa decrease in the population of said lower laser state resulting fromsubstantially any further increase in gas velocity is not substantiallyless than any decrease in population inversion resulting from theincrease in flow of said upper laser state out of said working region.17. In the method of liGht generation by stimulated emission ofradiation substantially throughout a gaseous active medium in a workingregion in a cavity having a gas inlet, a gas outlet and a longitudinalaxis the steps comprising: a. providing a gaseous active medium at apressure in a working region disposed in a cavity having walls forconfining the gaseous working medium that upon the production of freeelectrons in said medium said medium has ambipolar and thermal diffusionrates incapable of substantially damping local increases in freeelectron density in said medium, said medium having an upper and lowerlaser state; b. generating substantially throughout said working regiona substantially spatially uniform predetermined density of freeelectrons in said medium having an average energy insufficient toproduce a population inversion in said medium; and c. providing asustainer field for providing substantially uniformly throughout saidworking region a predetermined electron temperature effective toincrease the average energy of said free electrons without substantiallyincreasing said predetermined electron density by self-regenerativeionization, said electron temperature producing an average energy levelsufficient to support a population inversion in said medium.
 18. Themethod as defined in claim 17 wherein said medium is passed through saidcavity orthogonal to said axis and said sustainer field is an electricfield both orthogonal to said axis and anti-parallel to the direction offlow of said medium.
 19. The method as defined in claim 17 wherein saidsustainer field is en electric field both orthogonal to said axis andparallel to the direction of flow of said medium.
 20. The method asdefined in claim 17 wherein said free electrons are provided by means ofa discharge.
 21. The method as defined in claim 20 wherein saiddischarge is provided in the form of pulses whereby said spatiallyuniform density of free electrons is sequentially provided at discretespaced intervals of time.
 22. The method as defined in claim 21 whereineach of said pulses is provided for a time that is short compared to thetime required for said gas to flow through said working region.
 23. Themethod as defined in claim 21 wherein each of said pulses is providedfor a time of less than about 10 microseconds.
 24. The method as definedin claim 17 wherein said working region has a longitudinal axis and saidmedium is passed through said working region substantially uniformly allalong and orthogonal to said axis; and said sustainer field is anelectric field orthogonal to said axis and parallel to the direction offlow of said medium.
 25. The method as defined in claim 24 wherein saidfree electrons are provided by means of an electrical discharge.
 26. Themethod as defined in claim 25 wherein said discharge is seriallyprovided in the form of pulses and the energy added to the medium bysaid pulses is small compared to the energy added to the medium by saidelectric field.
 27. The method as defined in claim 26 wherein each ofsaid pulses is provided for a time that is short compared to the timerequired for said gas to flow through said working region and each ofsaid discharges is provided by a voltage that is high compared to thatof said electric field.
 28. The method as defined in claim 27 whereineach of said pulses is provided for a time of less than about tenmicroseconds.
 29. The method as defined in claim 24 wherein said mediumis passed through said cavity at less than supersonic velocity.
 30. Inhigh powered laser apparatus the combination comprising: a. gas supplymeans for producing a flow of a gaseous medium having a predeterminedvelocity and pressure and an upper and lower laser state; b. meansdefining a cavity including a working region for receiving said mediumfrom said gas supply means and through which said flow passes; c. firstmeans for producing a spatially uniform density of free electrons insaid mEdium substantially throughout said working region by ionizingsaid medium, said free electrons having an average energy insufficientto produce a population inversion in said medium; and d. second meansfor providing a sustainer field for controlling the electron temperatureof said free electrons in said medium to substantially uniformlythroughout said working region increase their average energy withoutsubstantially increasing the density thereof by self-regenerativeionization at said velocity and pressure and produce a populationinversion in said medium in said working region.
 31. In high poweredlaser apparatus the combination comprising: a. gas supply means forproducing a flow of an ionizable gaseous medium having a predeterminedvelocity and pressure; b. means defining an elongated optical cavityhaving a longitudinal axis, said means receiving said medium from saidgas supply means in a direction orthogonal to and extending along thelength of said axis and through which said flow passes; c. first meansfor ionizing said medium and produce a uniform density of free electronsin said medium, said electrons having an average energy insufficient toproduce a population inversion in said cavity; and d. second means forproducing an electric field in said medium containing said freeelectrons to increase the average energy of said electrons produced bysaid first means without substantially increasing the density of saidelectrons by self-regenerative ionization and produce a populationinversion in said medium during flow through said cavity at saidvelocity and pressure.
 32. The combination as defined in claim 31wherein said first means includes further means for providing adischarge in said medium for a time that is short compared to the timerequired for said medium to flow through said cavity.
 33. Thecombination as defined in claim 32 wherein said discharge is providedfor a time less than about 10 microseconds.
 34. The combination asdefined in claim 32 wherein said further means serially provides saiddischarge in the form of pulsed and adds energy to said medium in anamount that is small compared to that of said second means.
 35. Thecombination as defined in claim 34 wherein said second means furtherincludes means for providing the majority of the energy input to saidmedium.
 36. The combination as defined in claim 35 wherein said meansdefining said cavity includes gas inlet and gas outlet means and saidsecond means includes first and second electrode means disposedintermediate said gas inlet and outlet means, said longitudinal axisbeing disposed intermediate said first and second electrode means. 37.The combination as defined in claim 36 wherein said first means includesthird electrode means disposed upstream from said axis.
 38. Thecombination as defined in claim 37 wherein said second electrode meansare cathodes electrodes common to said first and second means and aredisposed downstream of said first electrode means.
 39. The combinationas defined in claim 37 wherein said first, second and third electrodemeans comprise rods spaced one from another extending across said cavityorthogonal to said longitudinal axis and disposed along the length ofsaid axis.
 40. The combination as defined in claim 35 wherein saidcavity includes end walls having means for passing a light beam throughsaid cavity parallel to said axis and downstream of said firstelectrode.
 41. In high powered laser apparatus the combinationcomprising: a. gas supply means for producing a flow of an ionizablegaseous medium having a predetermined velocity and pressure, said mediumhaving an upper and a lower laser state; b. means defining an elongatedcavity having a longitudinal axis, said means receiving said medium fromsaid gas supply means in a direction orthogonal to and extending alongthe length of said axis and through which said flow passes; said cavityincluding oppositely disposed end walls havIng means defining an opticalcavity parallel to said axis; c. first means for ionizing said mediumand produce substantially throughout said optical cavity a uniformdensity of free electrons in said medium; and d. second means forproducing substantially throughout said optical cavity an electric fieldin said medium containing said free electrons to increase the averageenergy of said electrons without substantially increasing the density ofsaid electrons by self-regenerative ionization and produce a populationinversion in said medium during flow through said optical cavity. 42.The combination as defined in claim 41 wherein said first means includesfurther means for providing a discharge in said medium for a time thatis short compared to the time required for said medium to flow throughsaid optical cavity.
 43. The combination as defined in claim 42 whereinsaid discharge is provided for a time less than about 10 microseconds.44. The combination as defined in claim 42 wherein said further meansserially provides said discharge in the form of pulses and adds energyto said medium in an amount that is small compared to that of saidsecond means.
 45. The combination as defined in claim 44 wherein saidsecond means further includes means for providing the majority of theenergy input to said medium.
 46. The combination as defined in claim 45wherein said means defining said cavity includes gas inlet and gasoutlet means and said second means includes first and second electrodemeans disposed intermediate said gas inlet and outlet means, saidlongitudinal axis being disposed intermediate said first and secondelectrode means.
 47. The combination as defined in claim 46 wherein saidfirst means includes third electrode means disposed upstream from saidaxis.
 48. The combination as defined in claim 47 wherein said secondelectrode means are cathodes electrodes common to said first and secondmeans and are disposed downstream of said first electrode means.
 49. Thecombination as defined in claim 47 wherein said first, second and thirdelectrode means comprise rods spaced one from another extending acrosssaid cavity orthogonal to said longitudinal axis and disposed along thelength of said axis.
 50. The combination as defined in claim 46 whereinsaid means defining said optical cavity comprises means for passing alight beam through said cavity parallel to said axis and downstream ofsaid first electrode means.